Broadband discrete-level excitations for improved extraction of information in bioimpedance measurements

The implementation of bioimpedance-based methods in implantable and wearable medical devices requires simple, cheap and low energy consuming measurement settings for enabling impedance spectroscopy at a wide range of frequencies. In the present paper, such a wideband bioimpedance measurement method is discussed, which embodies two-channel impedance measurement for monitoring of the frequency-dependent phase shift between the channels (phase spectrum). In addition, the improved resolution is achieved by employing comparative measurements by introducing the predetermined reference impedance into one of the measurement channels. The proposed and analyzed measurement system uses a binary excitation signal that simplifies signal generation and processing hardware and does not need sophisticated software—low-complexity devices can be designed this way. It is shown that in particular the binary chirp excitation has some essential advantages compared with its counterparts—the maximum length sequence and binary multifrequency excitations. The spectra of chirps of the binary chirp excitation, including their discrete-level modifications, are continuous and flat at the same time. Due to the independent scalability in time and frequency domains and very high chirping rate, the chirps are especially suitable as excitation signals for wideband spectroscopy of dynamic objects with changing impedances in devices such as implantable heart monitors, pacemakers and high-throughput microfluidic lab-on-chip-type devices for performing bioimpedance-based monitoring of cells and droplets. (paper)